The invention is based on a priority document EP 02 360 025.7 which is hereby incorporated by reference.
The present invention belongs to the field of cascaded Raman fiber lasers as well as to devices and systems containing such elements.
Raman amplification is known to be effective to provide a flat gain over a wide signal wavelength band when using several pump wavelengths. Broadband Raman amplification is therefore used to amplify signals over a wide wavelength band, and is thus of particular interest in WDM (Wavelength Division Multiplexing) optical transmission systems.
In order to provide the necessary pump wavelengths for the Raman amplification to be efficient (i.e. for the gain to be substantially flat), it is known to use a cascaded Raman laser having several output channels. The article xe2x80x9cA high-efficiency power-stable three-wavelength configurable Raman fiber laserxe2x80x9d, Mermelstein et al., Paper PD3, OFC 2001 discloses a WDM system using a multi-wavelength cascaded Raman fiber laser to provide pump wavelengths of 1427, 1455 and 1480 nm to a broadband Raman amplifier.
Cascaded Raman fiber lasers are known per se e.g. from document EP-0 651 479.
Such lasers comprise a length of optical waveguide, typically silica-based optical fiber, and means for introducing pump radiation of wavelength xcexp into the length of optical waveguide. The device further comprises n (nxe2x89xa72) spaced apart pairs of reflector means that define optical xe2x80x9ccavitiesxe2x80x9d for electromagnetic radiation of a predetermined wavelength, the cavities comprising at least a portion of the length of optical waveguide. Each reflector has an associated center wavelength xcexi of a reflection band, and two reflectors of a given pair have substantially the same center wavelength, such that the reflectors of a given pair define an optical cavity of length Li for radiation of wavelength xcexi essentially equal to the center wavelength of the reflectors of the given pair. With xcex94xcexi (i=1, . . . , n) being a length within the appropriate Stokes band associated with the fiber, xcexi=xcexixe2x88x921+xcex94xcexi (xcex0=xcexp). The reflectors have a high reflectivity, typically greater than 95%.
Therefore, the pump power can be converted, in a multiplicity of stages, to a power at a desired longer wavelength. The wavelength xcexi at a given stage is determined by the center wavelength of the relevant pair of reflectors, provided that the center wavelength is chosen such that the wavelength difference (xcex94xcexi) between the preceding stage (xcexixe2x88x921) and the given stage (xcexi) is within the Stokes band associated with the optical fiber. Such a laser further comprises a low reflectivity reflector for radiation of wavelength xcexs on the output side of the device, so that most of the power of wavelength xcexs is coupled out of the laser.
Thus, cascaded Raman lasers are based on Raman scattering, which is a non-linear optical process that involves coupling of light propagating through a non-linear medium to vibration modes of the non-linear medium an re-radiation at a different, typically longer wavelength. A photon is reflected back and forth in each optical cavity before undergoing Raman scattering that results in a photon of longer wavelength that then passes out of the cavity into the next optical cavity.
When a silica-based optical fiber is used as the non-linear medium, the maximum Raman gain occurs at a frequency shift of 13.2 THz, corresponding to a wavelength shift of about 50 to 100 nm for pump wavelengths between about 1000 and 1500 nm.
A cascaded multi-wavelength Raman fiber laser differs from a single-wavelength Raman fiber laser as described above in that it has several output wavelengths at the same time. It is based on the idea of splitting the Raman gain between several Stokes wavelengths having similar power levels in order to obtain several output wavelengths at the same time. To do so, the output reflectors of the pairs of reflectors having the desired output wavelengths as center wavelengths have a low reflectivity, so that most of the power of the desired output wavelengths is coupled out of the laser.
The known cascaded multi-wavelength Raman fiber laser described in the above article is configurable, i.e. the power at each wavelength can be varied by changing the reflectivity of the Bragg gratings used to form the cavity. However, the power dynamic range of the output pump channels of such a configurable multi-wavelength Raman fiber laser is not sufficient.
Indeed, when the outputs of the multi-wavelength Raman fiber laser are used as pump wavelengths for a broadband Raman amplifier for example, it is important that such pump wavelengths have precisely controlled respective powers, namely in order to ensure a similar power level on each output channel. This is not possible with the laser described in the above article.
Besides, it is also desirable to be able to select the required number of output wavelengths; in other words, it is desired to be able to use the three-wavelength Raman fiber laser described in the above article as a two-wavelength laser, or even as a single-wavelength laser.
It is therefore a goal of the present invention to provide a multi-wavelength cascaded Raman fiber laser having precisely controllable output power on each output channel, and also being suitable for use with less output wavelengths than the maximum possible output wavelengths.
To this end, the object of the present invention is a cascaded multi-wavelength Raman fiber laser adapted for emitting radiation of at least two distinct wavelengths xcexs1, xcexs2, comprising:
a length of optical fiber having input and output sections
means for introducing pump radiation of wavelength xcexp into said length of optical fiber
at least one pair of spaced-apart reflector means, defining an optical cavity belonging to said optical fiber, each of said reflector means having a center wavelength, the reflector means of each pair being located respectively at said input section and said output section of said optical fiber
at least one of said pairs of reflector means having its reflector means located at said output section, called output reflector means, having a lower reflectivity at said center wavelength than the corresponding reflector means of the same pair located at said input section, so as to emit radiation of said output wavelength out of said optical fiber
where the reflectivity vs. wavelength function of said output reflector means is such that the difference between the wavelengths of maximum and minimum reflectivity is at least 1 nm so that the reflectivity of said output reflector means at said output wavelengths is adjustable.
Preferably, said difference of wavelengths is at least 3 nm, and still more preferably at least 4 nm.
By choosing output reflector means having an adjustable reflectivity at the output wavelengths, it is possible to dynamically adjust the power of the output channels so as to precisely ensure a similar power level on each output channel, which is required when the laser is used as a pump for broadband Raman amplification.
Therefore, precise values of the reflectivity of each output reflector means are obtained which allows equalization of the power of the output channels.
In addition, it has been found that the power of the output channels also depends from the length of the fiber, the intra-cavity losses and the splicing losses. It is therefore all the more important to use adjustable output reflectors according to the invention as there are many parameters which may influence the output powers. The laser according to the present invention allows a dynamic control of the output power.
Preferably, the center wavelength of said output reflector means is adjustable. This is allows to xe2x80x9cde-couplexe2x80x9d the center and output wavelengths of the output reflector means; as the output power of the reflector means is maximum at its center wavelength, shifting the center wavelength of the output reflector means thus allows to adjust the output power.
In this context, adjustment of the center wavelength of said output reflector means can be made by increasing said center wavelength.
According to a preferred embodiment of the invention, the reflectivity vs. wavelength function of said output reflector means has a substantially triangular or Gaussian shape.
Adjustment of the reflectivity of the output reflectors means may be carried out, according to the invention, by adjusting the center wavelength of the reflection band of said output reflector. A preferred solution to reach such a goal is to choose a filter function of the reflector means having a shape which is not a step, and which is close enough to a triangle.
Indeed, in such a case, it is possible to gradually change the output power, which allows the maximum flexibility.
In a preferred embodiment for WDM applications, the laser comprises at least two pairs of reflector means and is adapted for emitting radiation of at least two distinct wavelengths xcexs1, xcexs2.
When the reflector means are Bragg gratings, this can be done by submitting said Bragg grating to strain (by heating or mechanically) in order to alter its transmission characteristics by e.g. changing its pitch. Changing the pitch of the Bragg grating leads to a shift of its center wavelength.
In such a case, the reflector means which are not output reflector means are such that the difference between their wavelengths of maximum and minimum reflectivity is less than or equal to 0.5 nm.
Advantageously, the reflector means other than the output reflector means have a full width at half maximum reflection bandwidth of at least 2 nm. This allows to have a maximum reflected power into the cavities.
The reflector means located at said input section and whose center wavelengths are equal to said output wavelengths can have a full width of 2 nm at a reflectivity value at 10 dB under the maximum.
In addition, said output reflector means have a full width at half maximum reflection bandwidth less than or equal to 2.5 nm, which is thin enough to precisely define the output wavelength.
The present invention also relates to an optical fiber communication system comprising a fiber laser according to the invention, further comprising:
transmitter means that comprise means for generating a signal radiation of wavelength xcexsignal 
receiver means spaced apart from said transmitter means that comprise means for detecting the signal radiation at xcexsignal,
optical fiber transmission means that connect said transmitter and receiver means
means for coupling the output radiation of wavelength xcexs1 of said Raman laser into said optical fiber transmission means.
The invention also relates to an optical Raman amplifier comprising a fiber laser according to the invention.